A useful estimate starts with three facts: the journey distance, the vehicle’s efficiency in a known unit, and the fuel price in a known volume unit. Keep those units attached to every number before applying a formula.

Start with the metric formula

For a vehicle rated in litres per 100 kilometres, multiply kilometres by L/100 km and divide by 100. A 500 km trip at 7.2 L/100 km needs 36 litres. For a round trip, double the distance first so every later result uses the complete journey.

Normalize MPG or km per litre

Kilometres per litre is inverted: 100 divided by km/L gives L/100 km. US MPG converts with 235.214583 divided by MPG, while UK MPG uses 282.480936. Never treat US and UK MPG as interchangeable because their gallons have different volumes.

Apply the matching fuel price

Multiply required litres by price per litre. If the station price is per gallon, first divide that price by 3.785411784 for a US gallon or 4.54609 for a UK gallon. Record the assumed efficiency and price because weather, traffic, load, and driving style change real consumption.